Preheat the oven to 350°. Lightly spray a 9x13 pan with cooking spray.
Lay chicken tenders in the pan. Sprinkle 1 tablespoon of the fajita seasoning on the chicken. Bake the chicken for 20 minutes, or until done. Drain the drippings. Cut the chicken into bite size pieces. Set both the pan and chicken aside. (This 9x13 pan will be used to bake the casserole, so don’t throw it into the sink just yet!) While the chicken is baking, work on Steps 3 through 5.
Heat oil in a large skillet over medium heat. Add peppers and onion and saute for 8 to 10 minutes. (The onions will begin to caramelize.) Remove vegetables from the skillet and set aside.
In the same large skillet used in Step 3, melt the butter over medium heat. Add cream cheese and melt, stirring often so it does not burn.
Add remaining fajita seasoning and ½ cup Colby Monterey jack cheese. Stir until Colby Monterey Jack cheese is melted. Remove the skillet from the heat.
Add the chicken and vegetables to the skillet and combine with the cream cheese mixture. Spread the mixture into the 9x13 pan..
Bake for 15 minutes. Sprinkle the remaining cheese on top and bake 5 minutes longer, or until cheese is melted.
Garnish if desired with the crumbling cheese and cilantro on top.